The present invention concerns dewatering blades for use in the forming and press sections of papermaking and similar machines, the blades comprising segments whose upper surfaces together comprise the fabric contacting surface of the blade, and in particular the bonding of one or more interlock members to the lower surface of the segments to interlock be secured within a channel within a base member.
Dewatering blades have been used for many years in the papermaking and similar industries to scrape or foil fluid from the undersides of fabrics so as to consolidate a mat or web, and to induce agitation in the stock carried by forming fabrics; many different designs are known and used. Although the present invention is described primarily in relation to the papermaking arts, the person skilled in the art will readily appreciate that the invention will find utility in all types of related continuous consolidation processes, such as sludge dewatering and filtration.
The fabric contact surfaces of these dewatering blades are typically comprised of carefully and precisely shaped segments of a ceramic or similar wear resistant material that are either bonded or mechanically attached in some manner to a base member. Typically the base member spans the full width of the machine and provides a firm base for the attachment of the ceramic segments; however, a sub-assembly system can be used, as described in our co-pending application U.S. Ser. No. 11/690,479. The fabric contacting surfaces of the segments are carefully shaped for engagement with the fabrics which pass over them in sliding contact. The machine side surfaces of the segments are typically shaped to fit a dovetail or other similar interlocking shape on the base member, as is well known in the art. FIG. 1 provides an illustration of the cross-section of a typical known ceramic segment of the prior art, showing the molded dovetail shape on the underside of the segment which is designed to mate with the fibreglass base member which is in turn mounted on a rail typically located on the top of a suction or other drainage box. This construction results in a substantial difference in thickness of the segment (measured from the upper to lower surfaces) at the location over the base member channel as compared to the areas upstream and downstream of same.
The prior art contains many examples of methods and apparatus to attach the segments to the base member; almost all rely on the basic dovetail and T-bar method disclosed by White et al. in U.S. Pat. No. 3,337,394. This patent teaches that drainage blades can be provided with a dovetail type recess adapted to fit over a T-shaped rail so as to mount them on the papermaking machine. The advantage of the method is that the blades are replaceable by sliding them off the rail and sliding a new one into position. The system also allows for accurate positioning of the blades in relation to the fabrics. Other examples are known.
Truxa in U.S. Pat. Nos. 3,520,775 and 3,647,620 teaches a means of adjusting the position of the blades relative to the papermaking fabric using a slot and pin type of arrangement. U.S. Pat. No. 3,743,574 to Walser et al. teaches a mounting system similar to White et al. U.S. Pat. No. 3,953,284 to Evalahti discloses a locking means for a T-bar type blade similar to that disclosed by White et al. Beauchemin in U.S. Pat. No. 4,004,969 discloses a removable wear resistant insert that is slidable into position to change foiling angles. U.S. Pat. No. 4,214,949 to Schiel et al. discloses a symmetrical blade adapted to be clamped to a support element by a leaf spring clip to provide interchangeable bearing surfaces. U.S. Pat. No. 4,544,449 to Arav discloses a mounting system for blades whereby the blades are secured to a “plank” by means of a series of pins which are welded to and project from the plank; the system is easy to manufacture and requires minimal machining.
U.S. Pat. No. 5,932,072 to Neun et al discloses mounting means in the form of “buttons” which hold the blades in position, replacing the traditional T-bar mounting means. Each blade includes a T slot on its lower side that is slid over the buttons; each button is comprised of a threaded cylindrical stem portion which engages with the machine frame and an upper cap.
U.S. Pat. No. 6,537,426 to Haunlieb discloses a connecting element or clamping means for releasably and adjustably securing a blade to the cover of a drainage device. Fasteners equipped with sleeves and located on the cover engage with angled slots in the underside of the blade mounting.
EP 1,127,186 to Jansson discloses a method of mounting a foil or blade whereby the blade is pushed into a stable operating position by means of springs which cause a locking member to engage clamping surfaces which hold the blade rigidly in position. An actuator is provided to allow the blade to be dismounted.
CA 1,235,010 to Fuchs discloses a blade comprised of ceramic segments which are comprised of upper and lower portions, the lower portions having projections which are arranged to be attached to a base member. The upper and lower portions are cemented together, but in an offset arrangement, such that each joint between adjacent lower portions is located partway between the joint between adjacent corresponding upper portions.
Currently, segments (typically ceramic) for use in papermaking dewatering blades are manufactured in one of three ways:    1. The ceramic is formed (i.e. cast) with an integral interlocking mechanism, such as a dovetail or tee shape, cast into the machine side surface of the segment. This interlocking shape is used in conjunction with a mating slot or protrusion in the base member to lock the components together. Epoxy is often used to facilitate this locking mechanism.    2. The ceramic segments are manufactured as flat rectangular components that are bonded directly to the base substructure with an epoxy suitable for use in a hot, wet, environment.    3. The segments are produced similar to (2) but they are cut with a groove that allows a steel clip to be inserted to aid in the interlock with the epoxy and the carrier base.
There are problems associated with all three of these current methods. In the first, the variation in the thickness of the ceramic (i.e. from the paper side surface to the bottom of the blade at its leading and trailing edges (T1 and T2) as compared to the interlocking portion at the base where the dovetail occurs and the ceramic increases in thickness as shown at T3 in FIG. 1) can lead to stress build up at the location of the profile change 17. This stress build-up causes stress cracks to form in the ceramic segments, subsequently leading to further cracking and ultimate failure of the segment. In addition, this locking means is costly to manufacture, and for various reasons can provide an unsatisfactory join between the base and ceramic segment.
In the second method, there is a strong possibility due to variations in the bonding process, or as a result of machine operating conditions, that one or more of the bonded ceramic segments may come loose from the base structure during machine operation, causing significant damage to the papermaking fabric and possibly the machine components. In the method disclosed by Fuchs, in CA 1,235,010, a very high accuracy of alignment is required, which is difficult to achieve having regard to the typical small sizes of the ceramic segments, and such construction is extremely difficult to repair in the event of damage to any of the segments.
In the third instance, the cutouts made in the bottom of the ceramic to accept the clips can also result in stress cracks forming in the ceramic segments, or they may reduce the usability of the ceramic due to the chance of breaking through into the cutouts.
It is thus desirable to provide a more efficient and cost effective means whereby the ceramic segments could be reliably and efficiently bonded to the base without danger of their being disconnected and causing damage to either the papermaking machine or its components, in particular the papermaking fabrics. It would also be desirable if the ceramic segments could be produced in a more reliable and efficient manner, in particular without the significant differences in thickness, so that stress cracking in the segments could be reduced to a great extent, and one segment could serve a variety of purposes in a given blade configuration.
Thus, there is a need in the filtration process industry, and in particular the papermaking industry, for a more reliable and cost effective means of attaching the ceramic segments comprising the fabric contacting surfaces of dewatering blades and the like to the base members upon which they are mounted. The present invention provides a simple but effective means of overcoming these and other difficulties relating to the attachment of the ceramic segments to the base member.